1. Field of the Invention
The present invention relates generally to the maintenance of machinery having a rotating shaft and, more particularly, to a system and method for measuring and analyzing electrical signals appearing on the rotating shaft of the machinery.
2. Description of the Related Art
It is well known that electric machines have some level of shaft voltage during operation. A number of specific sources may result in the appearance of voltage on a rotating shaft of an electrical machine. Variations in air gaps, windings, and metal (rotor and frame) result in a fluctuating magnetic field, which is cut by conducting material. This condition gives rise to a current loop among the shaft, bearings, and casing. For most machines this is not a significant problem. Very large machines and vertical machines are common candidates for these type of isometrics. In the past, this problem has typically been addressed by insulating the non-driver bearing, thereby breaking the current loop.
Residual magnetism can also be a source of rotating machine shaft currents. Even insulating the bearings of the machine may still allow a circulating current to exist in the bearings themselves. Another source causing voltage to appear on the shaft of a machine are electrostatic sources such as belts, charged particulate, charged oil, and sheet processes. Unless properly grounded, voltage will increase or build up until it can overcome the insulating material in the lubricant film and discharge through a bearing. Since the electrical characteristics of a bearing will vary with speed, lubricant, and clearances, the voltage magnitude or level required to "punch through" the oil film will vary. The higher the voltage magnitude, the greater the potential for damage to occur on the machine.
Although current is typically the damaging force for machine bearings, it is difficult to measure in field applications; especially levels due to capacitively based voltage peaking or spiking. Even when RMS based current measurements are low, damage from pulsed discharges on the rotating machine shaft can be severe. The application of current measuring test instruments tends to eliminate the capacitive charges, effectively hiding their presence. Relying on voltage measurements does reveal the potential for discharges to be occurring across the bearing/lubricant interface. When evaluating severity of the electrical signals appearing on a rotating shaft, several factors are typically considered, such as: a history of unexplained chronic bearing failure; examination of bearings of this or similar machines showing damage of an electrical origin; and typical voltage values measured on similar machines.
The standard practice for electrical machines with an induced voltage is to insulate the non-driver bearing, thereby breaking the loop that may exist from the rotor through the bearings and casing. If, however, the voltage is capacitive in nature, the voltage potential can build up or ramp until damaging levels of current pass through lubricant films in the machine or tachometer bearings. In such cases, a shaft grounding system may be required to diminish the voltage potential and to provide an alternate path for these high levels of voltage to discharge. It is known that grounding the machine shaft is the best way to prevent high levels of voltage from accumulating thereon and this application has been widely used in the industry. Although not all electrical machines experience these problems, variable speed electric machines (AC or DC), fans, blowers handling particulate, sheet processes, and some belt drives can be especially prone to these problems. In addition, variable speed drives can aggravate the problem.
As mentioned, machine faults due to the presence of shaft voltages and currents are often characterized by chronic bearing problems where no other logical root cause is apparent such as misalignment, overheating, or misapplication. Even when a shaft grounding system has been installed on the machine with the rotating shaft, the resistance between the grounding pick-up and shaft may increase, until it is no longer effective. Due to the nature of the voltage appearing on the rotating shaft, standard RMS voltmeters can be misleading when used to measure the voltage potential on the shaft because often the voltage is not at standard line frequencies, such as in the range of 50 or 60 hertz, may be erratic in frequency, and may occur as voltage peaks or spikes instead of being sinusoidal in nature. Therefore, a high frequency digital oscilloscope may detect spikes of 40 to 70 volts when an RMS measurement detects only 1 to 2 volts or less. It is a mistake to focus only on the RMS voltage and currents since voltage spikes due to capacitive type discharging can cause significant bearing damage in the machine.
It is therefore desirable in the art to have a system and method for measuring electrical signals on a rotating shaft of a machine that displays accurate measurements of RMS shaft voltage, peak voltage, and RMS current. It would also be desirable in the art to have a means of computing a Fast Fourier Transform on these electrical signals that appear on the rotating machine shaft and producing frequency spectrums as a function of time for easy readout by a system operator.